(i) Field of the Invention
The present invention relates to a process and an apparatus for producing high-purity chemicals for the microelectronics industry by dissolving at least one chemical gas in ultrapure water.
(ii) Description of Related Art
In order to produce ultrapure chemicals, such as aqueous ammonia, hydrochloric acid and hydrofluoric acid, it is known to use, respectively, "industrial"-grade anhydrous ammonia gas, gaseous hydrogen chloride and gaseous hydrogen fluoride and to purify them, in particular to purify them of their metallic impurities by scrubbing over a column packed with a solution saturated with the same gas in high-purity deionized water. A technique of this type is, for example, described in Patent Application WO 96/39265.
The technology described in the aforementioned patent application, which has marked an important step forward for allowing delivery to the integrated-circuit production site of the ultrapure chemicals which allow these ever smaller integrated circuits to be fabricated, still has, however, a certain number of drawbacks when a corresponding system is operated on a customer's site, for example an integrated-circuit fabrication ("wafer fab") plant.
A first problem encountered relates to the dissolving of the gas, which is accomplished, using the technology described in this patent, by injecting it directly into water. This results in a temperature rise and may cause sudden pressure variations due to intense stirring of the liquid. Furthermore, since the gas does not dissolve in the water instantaneously, this generates swirling in the liquid tank, which consequently means that the measurement of the titre of the solution is not always entirely correct.
Another drawback of the process described in this patent is that its operation is not continuous, thereby requiring, when the desired titre or desired concentration is reached, the content of the product container to be transferred to a storage tank (so-called batch process). Furthermore, the use of a heat exchanger as described in this patent application may possibly pose a problem when connecting the heat exchanger in the container for the product formed, in contact with coolant, which may be a source of pollution.
Finally, because of the always limited effectiveness of a mist eliminator placed at the top of a packed column, it is in some cases possible that an aerosol of a solution of the scrubbing liquid with the purified gas can in some situations pass through this mist eliminator, leading to a level of gas purity which may be limited.